Circuit protective devices and circuits



Jan. 7, 1964 R. L. HuR'rLE CIRCUI'Il PROTECTIVE DEVICE AND CIRCUITSFiled lay 1'1l 1960 3 Sheets-Sheet 1 faz Inventor:

ph L.. H rtle, bg /f/ dan A ort-neg.

Rai

Jall- 7, 1964 R. l. HURTLE Y 3,117,203

CIRCUIT PROTECTIVE DEVICES ARD CIRCUITS Filed lay 1'7.l 1960 3Sheets-Sheet 2 Pfg-2- F36- Ralph L.. H urble bgd ttor'neg.

Jam 7, 1964 R. L. HURTLE 3,117,203

CIRCUIT PROTECTIVE DEVIC AND CIRCUITS l :""Mm Ralph 1.. Huftle i /d MUnited States Patent O 3,117,293 CltkClUlT PRTEQTV t DlEt/lltES ANDCIRCUETCS Ralph l2. Hurtle, West plartnrd, Qonn., assigner to GeneralElectric Company, n corporation oi New York Filed May i7, Moi), Ser. No.29,629 3 Claims. (Cl. 20h-EB) rThis application is a continuation-impartof my application SN. 859,773 filed December 15, 1959, now abandoned,entitled Circuit Protective Devices and Circuits, and assigned to thesame assignee as the present invention.

li/ry invention relates to electric circuit protective devices,apparatus and circuits, and more particularly to current limitingdevices especially adapted for use in conjunction with means fordisconnecting or otherwise protecting electric load circuits andequipment. More specically my invention relates to an improved devicefor substantially instantaneously limiting the magnitude of apotentially destructive electric current, such as may arise from a shortcircuit or other abnormal circuit condition, and maintaining thiscurrent at or reducing it to a value which may be readily interrupted byother means.

By current limiting device l mean one having such action in response toa potentially destructive current that as result of that action themaximum instantaneous peak value of current which the device actuallypermits to llow (maximum let-through current) is considerably less inmagnitude than the maximum instantaneous peak value of current whichwould have been carried by the device had it retained its normalimpedance value (maximum prospective current), whether or not the devicesubsequently reduces the let-through current below such maximumlet-through value. This concept of current limitation is applicable toeither alternating current or direct current or direct current circuits,but since most utility power supply circuits are of the alternatingcurrent type, the concept is commonly applicable to the iirst half cycleof alternating current following the occurrence of a short circuit orother abnormal circuit condition. ln this sense current limiting meansthat the peak instantaneous current is never permitted to attain thefull maximum instantaneous value which the power source is capable fdelivering in the absence of current limiting action.

The constantly increasing consumption of electric power has engendered acontinual growth in the current supply capacity of power system. lnaddition, considerations of economy and emergency power supply have ledto the interconnection of many such large capacity power systems, withthe result that the available source capacity coupled to any onedistribution or utilization circuit at the output end of the system isenormous in relation to the current-carrying capacity of that circuit orthe current interrupting capacity of circuit breakers which may beeconomically applied to one such circuit. Of course, the entire largecurrent supply capacity of the source is not directly available to eachdistribution or feeder circuit, but is normally separated therefrom bythe limiting impedance of intervening transformers such as distributiontransformers. As systems grow in size, however, these distributiontransformers have become of increasingly large capacity, so that theyare capable of delivering very large short-circuit currents.

While circuit breakers used on transmission, subtransmission,distribution, feeder and utilization circuits are of course effective tointerrupt short circuit currents within their interrupting ratings, itis evident that ever larger circuit breakers are required as availableshort circuit currents increase due to system enlargement, systeminterconnection and increasing transformer capacity. This tends to bemore of a problem on the lower voltage distribution and utilizationcircuits than on the higher voltage Patented Jan. 7, 1964 transmissionand subtransmission circuits for the reason that in existing systems theratio of available current to normal load current is commonly higher onthe low voltage circuits than it is on the high voltage circuits.

Particularly in the lower voltage distribution and utilization circuitsof power systems, then, there has been an increasing tendency to requirecircuit protective devices, such as circuit breakers, to have everhigher short circuit interrupting ratings. The use of increasingly largeprotective devices, of course, presents an economic problem. As aresult, other means have been devised for protecting certain circuitinterrupting devices, such as circuit breakers and contactors, againstpotentially destructive currents in excess of their interrupting rating.For example, the now well-known current limiting fuse has been used inseries with resettable mechanical interrupting devices such as circuitbreakers and so coordinated with the circuit breaker characteristicsthat the circuit breaker interrupts current in a lower range of excesscurrents without fuse action while the fuse interrupts excess current ina higher range without circuit breaker action. Such an arrangement isshown for example in Patent 2,358,215, Darling. While such a seriesarrangement of circuit breaker and current limiting fuse is effective inpermitting the use or" circuit breakers or other mechanical resettableinterrupting devices on circuits having available short-circuit currentsin excess of the interrupting ability of the breaker, the combinationarrangement has the disadvantage that when the fuse does function, it isthe fuse which accomplishes the circuit interruption by fusing andself-destructing action. The circuit can then be restored only bycomplete replacement of the fuse. Such action, of course, eliminates oneor the principal desirable features of a circuit breaker,

insofar as a principal reason for circuit breaker use is to avoidcomponent replacement after each circuit protecting operation.

Thus while circuit breakers of large interrupting capacity are availableand even larger breakers can surely be designed, their use on relativelylow power circuits is often uneconomical where the ratio of availableshort circuit current to normal load current is very high. rthis is sobecause circuit breakers operate through the motion of mechanical partswhich have inertia, and even the fastest breakers do not actually eiectContact separation in less than one-half cycle of a commercialalternating current system. Therefore, as to its interrupting capacityand in respect to its ability to withstand high current momentarilywithout damage, a circuit breaker must be designed to accept one orseveral peaks of the maximum prospective short circuit current, and totolerate this high current value. While these problems of expensivecircuit breaker design have been avoided in certain applications byutiliz ig a series current limiting fuse to eiiect circuit interruptionunder certain high current conditions, such a fuse needs completereplacement after each fuse operation and thus sacrices the advantagesof ready resettability inherent in circuit breaker protective systems.

lt is evident, therefore, that in order to limit the size of circuitprotective and interrupting equipment, it is desirable to provide somemeans responsive to a short circuit or other excess current condition toseverely and substantially instantaneously limit the magnitude ofcurrent which actually ows in the circuit to a value very markedly lessthan the maximum current which the source is able to surply, suchlimiting means maintaining or reducing the li ited current without initself eecting interruption so that the limited current may be actuallyinterrupted by some other slower acting but resettabie device, it infact actua1 lterruption is needed or desired. A desirable currentlimiter of 'this type is, of course, not seit-destructive,

and is either rcsettable or self-restoring to its initial condition. Thecurrent limiting fuse mentioned above does, of course, have d sirablecurrent limiting action and quick reponse, but is not seit-restoring forthe reason that the current limiting action, once initiated, destroysthe fuse and etlects circuit interruption by fusing action. It isevident, of course, that conventional im edance elements such asresistors and inductors provide automatically repetitive current tingaction. Such conventional expedients for current limitation have, ofcourse, been considered, but they have not proven economical orpractically feasible because of their necessarily excessive size andcurrent-carrying capacity.

Accordingly it is a principal object of my invention to provide anautomatically operable device of a resettable or self-restoring type forsubstantially instantaneously limiting the magnitude of an excessiveelectric current without in itself interrupting that current.

A more particular object of my invention is to provide a self-restoringchangeo-state device for severely and substantialiy instantaneouslylimiting excessive currents of short circuit magnitude without effectingcurrent interruption.

Still a further object of the invention is the provision of a reusablecurrent-limiting device which functions by developinr7 and sustaining ahigh pressure arc having a high potential drop without in itselfquenching the arc.

lt is another object of my invention to provide a device of theforegoing character which, in addition to limiting the maxi num.stantaneous magnitude of current, subsequently reduces appreciably thecurrent magnitude and regulates the current to a substantially constantreduced value.

Still another object of my invention is to provide a curreutlimitingcircuit protective device which, partly because or its comparativelysmall heat sink, limits potentially destructive currents to much lower1etthrough values than does a comparably rated conventionalcurrentlimiting fuse.

in another aspect it is an important object of my invention to provide,in combination with a current interrupting device of the switch, circuitbreaker, or contactor type, a current limiting device or the resettableor selircstoring type which is automatically and substantiallyinstantaneously operable to limit the magnitude of potentiallydestructive currents, such as short-circuit currents, and 'therebyconsiderably reduce the current withstand and current interrupting dutyimposed upon the circuit opening device.

ln particular, the invention has for its object the provision or" animproved and substantially instantaneously operable current limitingprotective device for power circuits which maintains electricalcontinuity therethrough during and after its current-limiting action,and further contemplates the circuiL coupling of such a device with relosablo current intcgrupting means, such as a contactor, switch, orcircuit breaker, to eliectively interrupt excessive currents of shortcircuit magnitude without undue burden on the interrupting element.

ln carrying out my invention in one form, I provide a body orexceedingly strong and inelastic non-porous insulating material havingone or more capillary passages formed therein and filled with avaporizable metal each to form a conducting filament, pluralcapillaries, ii provided, being disposed in substantially parallelspaced-apart physical relation and connected in parallel electriccircuit relation, such device functioning upon the conduction of currentot large magnitude to develop within the capillaries sustained highpressure arcs having high voltage gradients.

ln a preferred embodiment of my invention l provide an exceedinglyinelastic enclosure capable of withstanding explosive internal forcesfor significant periods of time, such, for example, as a full half cycleof commercial alternating current. Within that enclosure there isprovided an insulating body formed of a non-porous, high density ceramichaving one or more capillaries extending therethrough. rThesecapillaries are filled enduro-end with a conducting material which inone oi the preferred embodiments is a eutectic mixture of sodium andpotassium. Opposite ends of these conductive material-iilled capillariesare in contact with terminal members insulated from one another andextending to the outside of the enclosure. The contents of thisexplosion-proof enclosure are preferably placed under high initialpressure.

The conductive material in the capillaries is capable of carrying apredetermined amount of current without substantial change in itselectrical properties. However when the device is subjected to excessivecurrents of sufficient magnitude to vaporize all or part of theconductive material within the capillaries, an extremely high pressurehigh resistance steady-state arc is established in each capillary,limiting and reducing the current dow therethrough to levels which mayreadily be interrupted by conventional switching means.

ln one presently contemplated combination aspect of my invention, Iprovide a circuit including a device of the foregoing character sointerconnected with a circuit reaker or switch that the current-limitingdevice limits potentially destructive currents to a magnitude within theinterrupting ability of the circuit breaker or switch. Such circuitbreaker preferably is provided with thermal as well las magnetictripping elements, both of which are coordinated with the currentcharacteristics oi the limiting device. ln the event or" large faults orshort-circuit current through the circuit breaker, the current-limitingdevice acts substantially' instantaneously to limit and reduce thelet-through current and to permit the circuit breaker itself tointerrupt the reduced current.

ln appearance the contents o lthe current limiting device itselfresemble supercially some types oi fuses except that, as will becomeapparent, in the event that the conduct-ive material in the capillariesvaporizes it continues to occupy substantially the same volume as in itssolid or liquid state. Actually when currents of values in excess of apredetermined amount are passed through the device, the conductivemateriel in the capillaries vaporizes in situ. Since the entire assemblyis exceedingly inelastic and non-porous the vaporized conductor w.-. nthe capillaries develops enormous pressures. Consequently, although aconductive path through the current limiter is maintained, the arc wir'h is sustained in the device is a high voltage gradient, currentlimiting arc. The higher the pressure becomes, the higher the voltagedeveloped across the device and the more eflicient the current lin r ingaction. Because ot the small heat sini; provided by the conductivecapillaries the yresponse of this current limconductive material in thecapillaries is held, the conductive material continues to occupy thesame volume with substantially no oscillating expansion and contractionpermitted. And b cause the conductive contents are not blown out or thecapillaries the device may be used repeatedly without even being reset,

The details of this invention as well as additional obiects, advantages,and applications thereof will be more readily perceived from thefollowing description together with the accompanying drawings wherein:

FlG. l is a cross-sectional view of a current limiting deviceconstructed in accordance with this invention;

EEG. 2 is an enlarged plan view of lie insulator bodyV portion of thedevice of HG. fl;

PEC-S. 3, 4, 5 and 6 are cross-sectional views showing various alernative modes of const "action for current liml ing devices embodyingmy invention;

FG. 7 is a side cross-sectional view or a circuit breaiter and currentlimiter combination combined in a unitary mounting for series-connectedcoordinated protective operation;

8 is a graphical representation of the time current 5 characteristics ofa thermal-magnetic-trip circuit breaker, of certain current limitingfuses, and of a current limiter constructed in accordance with thepresent teachings; and

FIG. '9 is a reproduction of an oscil'lographic trace illustrating thecurrent limiting eil-ect on short circuit duty of devices embodying thepresent invention.

FIGS. l and ll are diagrammatic views of a current limiter embodying myinvention and connected in another circuit arrangement with a switchingdevice, FIG. l0 showing the switch in closed circuit position and FlG.ll showing the switch ope FIG. l2 s an oscillographic trace of currentthrough the limiting device -in the circuit of FlGS. l04 and ll.

Turning now to FlG. l there may be seen a preferred embodiment of acurrent limiter constructed in accordance with this invention having avery strong steel housing Il@ at the center of which are the elementswhich provide the current limiting action. The housing is shown as atubular body of any desired outer cross sectional contigura-tion andhaving a cylindrical axial opening lin which the current limiter elementis disposed as a core in radially spacedepart relation with the housing.The current limiting element comprises body lll of dense non-porousceramic material, herein illustrate as a disc, and having a number ofsubstantially parallel centrally located capi-llsries l2 each filledend-to-end with a metallic conducto-r i3. Additional details concerningt-e ceramic body lll, its capillaries l2, and the metallic conductingmaterial TLS are to be set forth below aiter a description of thegeneral organization and operation of the device. Meanwhile `it shouldbe kept in mind that the severest physical demands are to be made ofthese parts and that, in order Ito obtain the maximum benefits from thepractice of this invention, the highest grades of materials should beemployed.

Opposite ends of the conductor-filled capillaries l2 are in electricalcontact with reservoirs le and l5 also filled with the same conductivematerial. These reservoirs nre contained within internal terminalconducting members le land i7 whichl are bonded at boundaries lli to theceramic disc ll 'to complete a rigid enclosure for the couductonilledcapillaries. rllhe particular embodiment shown is designed :toaccommodate within the capillaries l2 and the reservoirs l/i and yl5 ametallic conductor which at ordinary temperatures is liquid. To containthe liquid conductor within its capillaries and the reservoirs and toadjust the total enclosed volume therein to coincide 'i the volumeconductive material employed, each internal Aternrinal member lo and l'has inserted therein with a lioht interference press llt ya plug i9. Auannular gasket or `liquid seal 2d is interposed between each terminalmember lo and its plug it?. Each plug contains a s iv member 251 axiallyadiustable at the time the reservoir and capillaries are filled toexclude vapor locks or air bubbles which can cause the malfunction ofthe device. Filling of the reservoirs and capillaries is preferablycarried out in a vacuum chamber, after which air under normal pressuremay be admitted to the chamber, thereby forcing the liquid conductorinto any unlled voids.

ln electrical Contact with the outer ends or" the internal terminalmembers lo and i7 are a pair of terminals Z2, and 23 respectively. Theseterminals have iianges 2li and 25 n their irincr ends and internallythreaded bores and 25" on their outer ends accessible from the outsideof the steel enclosure. Through these threaded bores electrical IContactmay e made between the current limit er device and the circuit withinwhich it is employed.. The enclosure lll comprise stout steel casing 3dprefcrcfbly in the `ferm of a rectangular solid with a cylindrical axialbore Sil containing the current-liniiting elements. @n either end of thecasing 'du are provided steel caps and clamped by four machine screws3d, of which two are visible FlG. l, which extend through the casing Eiland the caps and 33. ti/hen it lis realized that the device shown inlilG. l is drawn to scale, it becomes 6 apparent that the device isdesigned to withstand very large pressures.

Obviously some arrangements must be made to insulate portions ot thecurrent limiter from its steel enclosure. To that end terminals 22 and23 have about them insulating sleeves 35 and 3io respectively,preferably formed of a suitable epoxy resin and having large external toprovide a generous oversurface leakage path to prevent creep tracking.Backing up the internal flanges Zd' and 25 of terminals 22 and 23 are apair of insulating spacer assemblies each including two ceramic rings 37with a dense packing member 3S therebetween. These spiace assemblies aid`in insulating the terminals 22 and 23 from casing Si?, and `they serveto maintain the terminals within the enclosure by transmittingcompressive forces of the clamped caps 32 and 33 to ehe shoulders 2d and25 of the terminals.

The internal terminal members are spaced from the Walls of bore 3l byreason of the fact that they t tightly within cup-shaped collars 39 ofterminals 22 and 23. The ceramic disc and its internal terminal membersare insulated from the casing 3d by a hydraulically pressurized oilwhich lls the space between these elements and the inner surfaces of theaxial bore 3l within the casing. ln order to equalize the hydraulic oilpressures between the ends and the sides of internal terminal members 16and '17 the collars 39 are castled, or deeply notched longitudinally,about the periph-ery of their reduced end portions. The oil is preloadedby means to be described in order to place a high initial pressure onthe internal parts of the current limiter, thereby to aid in holding thelimiter assembly together while under the inlluence of the potentiallydestruction forces which are developed during current limitingoperation. I have obtained hydraulic pressures of as high as 300,00()pounds -per square inch With a construction of this type.

At the pressure gradients resulting from pressures of this magnitudemany materials which are ordinarily rigid, including some metals, mayflow like warm butter. It iis therefore to be expected that in order tocontain these pressures the various packing materials shown in FIG. l,such as packing member 3d, are very stili, not at all rubberlike, andthat the tolerance surrounding these materials must be very close.

I have shown a threaded opening 4l. communicating With the axial bore 3lfrom the `outs-ide of the casing to provide for iillinig the casing withoil. The bore dl, through which an excess of hydraulic iiuid isintroduced, is provided with a threaded bolt ldto `force out excessfluid and to close the casing. Cap 32 is then tightened down causing apiston-like stroke of rings 3'7 and member 3S to force the fluid intohigh compression.

As described, the current limiter shown in FIG. l has a conducting pathextending from one terminal 22 to the other terminal 23, thus conductingall currents in parallel circuit relation through the conductor-iilledcapillaries l2 in the ceramic disc lll, best seen in the enlarged endView of FlG. 2. As was mentioned in the brief summary of the inventionpreceding the present detailed description, currents Athrough thesecapillaries in excess of a predetermined ymagnitude actually occasionthe vaporization ot most of .the metal Within the capillaries. Since noprovision is made for the escape of the vaporized metal, thetemperatures 'and the pressures which develop in the capillaries arevery large, the temperatures sometimes even exceeding those at theIsurface of the sun. The fact that the conductive material in thecapillaries actually vaporizes Without substantial change in volume is adifiicult one to grasp, but an important feature of this invention. Verylittle is known of `the characteristics of a vapor in this state, whichis said to be its critical point, i.e., that condition of a vapor atwhich no further increase in pressure can cause it to condense.Moreover, the general gas laws concerning pressure, temperature, andvolume do not apply to a super-heated vapor of this nature.

The entire enclosure should be sufliciently rigid to prevent anysubstantial exure or elastic deformation due to the very large internalpressures generated in the capillary apertures. If such liexure werepermitted the conductive material Within the capillary `apertures wouldalternately expand and `contract with a detrimental oscillatorymovement. In fact in those circuits for which this device is intendedthe large electrical currents through the device develop a 'highpressure are, the potential drop across which results in a rapid andcontinuous current limiting action. The current limiting highvoltagegradient across the arc, being of the order of a thousand volts perinch, depends upon the pressure in the arc and it is therefore importantto contain the pressure developed instead of to relieve it. Oscillationof the conductive material in the capii-lary apertures might interruptand reestablish the arc periodically. At the very least it would `causean oscillation of pressure within the device which, by periodicallyreducing the internal pressure would periodically inhibit the currentlimiting action. Consequently, the enclosure is made Asufficiently rigidto prevent oscillatory movements of the conductor in the capillarieswhich would cause substantial changes in volume of the conductor.

in accordance with an important feature of the invention, the currentlimiting device does not itself cause the current to goto Zero. For ifthe arc were to be quenched, the conducting capillaries Wouldimmediately become, through condensation of the metal vapor, aconducting path once again for the establishment of a new arc and thecurrent limiting function would be adversely affected consequently,during current limiting action, the device consumes substantial amountsof energy. its ability to dissipate these energies without seriousdamage to itself is one of its significant characteristics.

It is very important that the number, length, and diameter of Itheconductor-llled capillaries yin the ceramic body of the current limitingdevice be selected to provide optimum operation of the device withregard to the electrical characteristics of the circuit in which thedevice is to be used. Considerations involved in the determination ofthese characteristics include the following:

A. Current regulation-ln accordance with the invention the currentregulating action of the device by which the current through it issuppressed to a desired level without extinction is :achieved primarily`by adjusting the length of the ycapillaries in accordance with the linevol*- age of the system in which the device is intended to be used andthe desired magnitude of the regulated or suppressed current desired tobe permitted. This adjustment is possible because at any given currentlevel the voltage drop of the lilamentary high pressure arc, over andabove the relatively small portion (about volts) caused by the cathodeand anode voltage drops, varies with the ylength of the are. I,therefore, select fa length for such conductor laments which provides atotal arc voltage drop which, in the current suppression portion of thedevices characteristic, is less than the line voltage by an amountsutlicient to provide the desired level of suppressed current.

The magnitude of the desired suppressed current, in turn, is determinedprimarily by the amount of time which the current may be expected toremain at the suppressed level before extinction by other means, and bythe ability of the current limiting device to dissipate the energydeveloped during this time. In general, I prefer to maintain the currentduring this period as low as practical. On the other hand, in certaininstances it may be found desirable to tolerate a moderate amount ofcurrent at this time in order to permit shortening the capillaries.

B. Power loss when not current limiting-The normal or non-arcing powerloss associated with a given capillary of a given diameter is directlyproportional to its length. For this reason, it is desirable that thecapillary length be maintained as short as possible consistent withother factors. The shorter the length, of course, the higher Will be therequired voltage gradient through the capillaries in the arcingcondition. In accordance witn the invention, I subdivide the requiredcross-section of conductive material for a ygiven normal or continuouscurrent rating into a number of relative small diameter tilarnentaryconductors, each having a relatively high voltage gradient in the arcingcondition, and a relatively short length.

C. Arc energy absorption-The thermal eiiiciency of the device, i.e., itsefliciency in dissipating the internally developed energies withoutserious damage to itself, depends on a number of factors, chief amongwhich is the ratio of surface area of these capillaries to theircontained voltune. The combined surface area of all the capillariesshould be as large as possible for any given current rating, for on thissurface area depends the rate at which energy released within thecapillaries may be dissipated. The capillaries should therefore be assmall as possible and should be correspondingly numerous to make up thecombined cross-sectional area required.

l). Pressure.-While actual measurements of the pressure within thecapillaries are not as yet available due to limitations of presentpressure measurement techniques, it is believed that the subdivision ofthe conductive medium into ne capillaries also results in lowerpressures Within such capillaries for any given current density, due tothe improved energy absorption rate provided thereby. Such reduction inpressure is, moreover, not achieved at the expense of reduced voltagegradient. Thus, energy absorption or cooling of the arc causesdeionization thereof and increases the voltage gradient therethrough.

At the same time, nevertheless, my observations indicate that thevoltage gradient of the arc is improved by increased pressure. Thus, itappears that the voltage gradient across the capillaries is a positivefunction both of pressure and of the energy absorption rate. Theprovision of relatively line capillaries accordingly contributes tohigher voltage gradients by providing better energy absorption at agiven pressure.

E. Manufacturing ec0n01ny.-The number and size of these capillariesshould be consistent, of course, with reasonable economy in themanufacture of the device. Although I have made capillaries for thispurpose as small as .0G93 inch such capillaries are very difficult tomake and to fill with a conductive material. I have successfully lilledcapillaries as small as .001 inch, but have found that a reasonablecompromise with all factors involved is to employ capillaries of about.0135 inch. Such capillaries are small enough in diameter to give afairly high ratio of surface to volume, but large enough to be drilledand filled with comparative ease. With dimensions of this size thethirty-seven capillaries shown in the ceramic disc of FlG. 2, whenfilled with mercury, are capable of carrying currents of amperescontinuously without special cooling provisions and of limiting currentssigniiicantly in excess of this value. The distance between thecapillaries is not too critical, but I prefer to group them in thecenter of the disc as shown so that the surrounding portions of theceramic disc provide a substantial amount of reinforcement.

It has already been stated that maximum benefits of this invention arerealized when high grade materials are employed in its construction.rlhere are no known materials in the universe, however, which cannot beeroded by a high temperature, high pressure arc. Nevertheless thesubdivision of the arc into a number of very small parallel-connectedsections results in the radiation and conduction of energy away from thearc sections with such speed that the capillary surfaces of the ceramicdisc, for example, are but little affected by the arcs. This is not tosay that the disc has an unlimited life, for it is in fact erodedsomewhat each time a high temperature arc is produced within it. But theconstruction 9 of the device minimizes the destructive etiects, andpromotes a long useful life so that the device is capable of repeatedcurrent limiting operations.

To further minimize the destructive effects of the current limitingarcs, l prefer to employ as the material for disc ll a very densepolycrystalline ceramic formed principally of fine-grain high-purityaluminum oxide powders which are pressed at room temperatures and tiredat teniperatures which are higher than usual for ceramics. Thismaterial, a product of modern scientific research, is available inlimited quantities at date of this writing from General Electric Companyunder the name Lucalox. Among its characteristics which suit itparticularly well for use in devices constructed according to thepresent invention are its extremely high physical strength, itsphenomenal resistance to high temperatures, and its rcmai'kaaletrsnslucence. These advantageous characteristics are due in large partto the fact that the microscopically sinall pores or bubbles normallyfound in ceramic materials are virtually absence from Lucalox. Thetranslucence of this material contributes further to the eiiectivecooling of the current limiting arcs, by inipi'oving radiation oi energythrough the ceramic disc. to a surrounding medium. l do not wish togivethe irnpression that the practice of this invention necessitates theuse of this particular cerainic material, for other dense insulatingmaterials may also be employed, ont it is should be understood that thismaterial is, in my experience, the most suitable of those available.

Another contribution to the tl mal efficiency of this device is affordedby the rela ivf-"y massive inten. i terminal s lo and lil? together withtheir enclosed reservoirs of conductive material, both of which aredisposed very close to the high temperature arcs in the capillaries. Eycontrast, current limiting fuses customarily have long perforatediusibie 'lllainents, the. major portions of which are remote from themore massive end terminals. And because the fusible elements areordinarily surrounded by quartz sand, which is a very effective thermalinsulator, the small arcs formed by fusing and vaoorization of thefilaments at their periorations do not cool as readily nor do theydevelop potentials nearly lign as the arcs developed in current limiteraconstructed according to the present invention. Furthermore, current l`fuses, because of their relatively poor thermal etiiciency are subiectto a fault known as nuisance tripping; that is to say, currents at ornear theiriated current levels may, because of the effective insulationof filter, a ps ilow of the fusible elern nts with a consequentundetected change in the current 11ncharacteristics. As a result, latercurrents witnin the original current rating ot current lim iuse maycause the inse to interrupt unnecessarily. These faults, common in ci'rerL limiting tuses, are substantially overcome by the presentriverition largely because of its bien thermal eliiciency.

`lidaterials mentioned so lar in this description to till thecapillaries 1.2 have been mercury and e tecti niixture oi sodium andpotassium. Tiese not the only such materials which may be enti eyed, noris it absolutely essential tnat conductive materials within thecapillaries be a liquid at ordinary temperatures. l have preferred touse liquid metals, however, because such metals should be capable ot'producing a faster current limiting action than should solid metalssimilarly employed. This is a consequence ol thc tact that in raisingthe temperature of a given amount of any substance a delnite energyinput is required merely to change its state from solid to liquidwithout even elevating its temperature. This heat ot fusion, as it iscalled, represents a denite amount of energy which must be put into anygiven quantity of the material beiore the temperature or" the conductormay be raised further toward its temperature oi vaporization.Neccssarily, a certain amount ot time is required to perform thefunction of melting a solid metal. In the current limiting devicedescribed herein, the extra time re quired, though small, would delaythe current limiting operation. A metal already in its liquid state thushas a head start, so to speak, on a solid metal and should usually becapable ot reducing overload or short circuit currents more quickly thana norrially solid metal. Furthermore it is generally true that metalswhich are liquid at ordinary temperatures also have lower boilingpoints, and this too should contr' te to the rapidity with which theyvaporize and limit current in such devices. ln addition, by its nature aliquid metal is easier to handle in filling small spaces such as thosein the capillaries. Nevertheless, despite these factors I havesuccessfully used silver as a tlller for the conductive capillaries.

ln selecting a conductive material to be employed within thecapillaries, other considerations are involved than its physical stateor its conductivity. For example its chemical nature should also be keptin mind. It is to be noted that very active rnetals, such as lithium,sodium, and potassium, may combine chemically with the ceramic ot whichdisc is formed. At ordinary temperatures and in the absence oi catalyticmaterials oxidation-reduction reactions of this nature may proceed soslowly as to be negligible. However, under the iniluence of very hightern cratures and pressures the reaction may be considerably acceleratedwith a consequent gradual chemical deterioration oi the ceramic disc.This, of course, would alter the size of the capillaries, and wouldconsume some of the conductive material. it would therefore have animportant effect upon the electrical characteristics of the device.Shelf life is also a factor to be considered, for even if the currentlimiter is not actually in use or it is in use continually but is nevercalled upon to limit excessive currents, a very slow but progressivecherni reaction may alter its electrical characteristics to thedisadvantage of the user. Hence, a less active metal such as rver ollcrscertain advantages which cannot be ignored. For myself l have preferredto employ mercury as a reasonable compromise, but l wish to beunderstood that this is purely an illustrative prefera ce, and that manyother vaporizable conductors may be emuloyed without departing fromthese teachings, the selection of a particular conductor depending onthe exigencies.

Although the materials employed for the construction of the disc il andfor the conductive material within its capilleies are the most critical,the selection of other materials used in the current limiter should alsobe carefully made. For example, the internal terminal members lo and if?are prei" 'ably formed of a material whose temperature coeicicnt ofexpansion matches as closely as possible that or the ceramic dise lill,For this purpose l prefer to employ a solid conductive material formedof a mixture or ceramic and metal and identied in the trade as cerinet.in View of the extremely high pressures contemplated in operation andthe consequent requirements for strength and rigidity in the entirestructure, it is essential that any ceramic bond, such as that at theboundaries i8 of FIG. l, be of the highest quality. Various suitablebonding techniques are known in the art. @ne such method will be desc'ibed hereinafter, but it will be understood that the bonding methodselected must be compatible with the materials of the ceramic disc itsbonded terminals. The gaskets 2l and 3S may be of a relatively stillsilicone rubber it the tolerances about them are very close. G-rings oftie same material may be inserted under the heads ot the screw membersZtl as a further aid in sealing the reservoirs ltanf l5 after lilling.However, for the gasket 2S inserted under the head of the bolt 44, lprefer to use a soit metal such as copper.

At FIGS. 3, 4, 5 and 6 l have shown current limiter elements embodyingmy invention in various other structural forms. All these currentlimiter structures, inclu ling that of EEG. l, may be used with orwithout the ll hydraulic preloading described connection with llG. l. lnthe embodiments of FlGS. 3, 4, 5 and 6, parts corresponding to likeparts at PEG. l have been assigned the same reference numerals.

The current limiter shown at FIG. 3 comprises a central disc-shapedinsulating body li of dense, nonporous, vacuum-tight, highly refractoryceramic material, as described hercinbefore. As will be notedhereinafter, other refractory materials be used for the disc Ill if theyprovide the required strength, temperature resistance and thermalconductivity. The disc il is provided with a plurality of small-diameterparallel holes or capillaries l2 extending therethrough in the centralregion thereof. A pair of elongated terminal members 16, 17, havingflanges ida, T70., respectively, of substantially the same diameter asthe disc Ill, are provided, which are integrally bonded to the oppositesides of te disc lll by brazing, in a manner to be described. ln thisstructure of FIG. 3 the terminals le, i7 are formed of stainless' steel.A pair of additional ceramic members or backup members, Si, 52, are alsoprovided of the same material as that of disc ll, bonded to the outersurfaces of the flanges llcz, la, for a purpose also to be describes.

The steel terminal members i6 and It? are each provided with arelatively shallow central circular recess ld and l5, res ectively, ofsufficient diameter to encompass all of the ends of the capillaries l2,for a purpose to be described.

To facilitate filling of the assembly with liquid conductive material ina manner to be described, the terminal member l? is also provided withan axial opening extending from the recess le' to the outer end of theterminal and comprising a first relativelyi small diameter portion lSaand a second relatively large diameter portion 15b, which is tapped toreceive me shank of a closing bolt 2li. The axial opening also includesa further enlarged portion or counterbore Ztl-a to receive the head ofthe bolt 2li and providing a circular shoulder or ledge 29h adapted tosupport a sealing ring 2li which is clamped thereagainst by the head ofthe bolt 2d.

Thus the recesses le, l5, the capillaries l2, and the openings 15a andlib constitute a sealed chamber which is closed by bolt 2li and seal 2l.'ihis chamber, in the illustrated embodiment, including all portionsthereof, is completely filled with mercury f3, the portion of themercury filling the capillaries Il?. thereby forming a plurality ofrelatively ne iilamentary conductors within th ceramic disc lllinterconnecting the relatively large conductive assemblies at each sideof the disc il comprising the terminal members lo and 17 and the portionof the mercury fill therein.

Following filling of the chamber in the manner described above for FlG.l, the bolt 2li is inserted, there beingstill at this time mercury inthe major portion of the opening lh. insertion of the screw displaces aportion of this mercury and forces it out of the terminal l'l around thethreads of the bolt until the head of the bolt contacts the O-ring seal2. Further tightening of the bolt Ztl compresses the seal 2l slightlycreating a vapor-tivht seal and placing the mercury under a slightcompressive force.

ln the form shown in FiG. 3, in which the disc il comprises aluminaceramic and the terminals lol and l? are of stainless steel, theterminal members are of t'ie flanged construction as shown and describedparticulary for the purpose of facilitating the bonding of the terminalsto the alumina ceramic disc ll to provide a vaportight, high-strengthbond.

The parts referred to may be bonded together by various methodspresently known to the art. A method which has proved particularlyeffective, is as follows.

The ceramic disc lll is prepared by lapping the bonding surfaces thereofto a high degree of smoothness, after which it is washed, dried, andleated in air at 1G09 dcgrces centi,D e for one hour. rEhe terminals 'l2lo, l? are ground to a high degree of smoothness at their bondingsurfaces, washed, dried, and heated in vacuum at 100() degreesccntigrade for one hour, and permitted to cool at room temperaturebefore removing from the vacuum.

The bonding surfaces of the ceramic are next coated with a thin evencoat of a mixture comprising 66 parts of amyl acetate to l() parts ofindopol polybutene, and allowed to dry. These surfaces are then coatedwith a layer of titanium hydride powder, the coating having a density ofabout .00941 gram per square inch of surface.

A brazing shim is then prepared comprising a thin flat ring ofsilver-copper eutectic alloy, thc amount of alloy used being equal inweight to .616% of the amount of titanium hydride used. The brazing shimis also cleaned by washing in acetone, drying, and heating at 400V C.for 2G minutes in a vacuum, being allowed to cool before removal fromthe vacuum.

rl`he parts are then assembled in the desired relation with a brazingshim between each pair of surraccs to be joined, and held by a suitablefixture, not shown, so that an initial loading force of 30 lbs/in.2 isapplied to the areas to be brazed. A vacuum of less than 1%-4 of Hg isdrawn on the assembly, and it is heated to 500 degrees centigrade. Whenthe titanium hydride is transformed by this heat to titanium, liberatinghydrogen, the temperature is increased to 1G09 degrees centigrade untilthe silver-copper eutectic brazing shim melts, the temperature beingcontinued at this value for one minute thereafter. The assembly is thenallowed to cool in the vacuum to room temperature.

Since `the alumina ceramic material of the disc lll has a substantiallydifferent (lower) coefficient of thermal expansion from that of thestainless steel metallic material of the terminal members 1.5, f7substantial shear stresses are set up at the bonded interfaces. 'l'eseforces tend to bend the bonded components in what is known as a bimetalaction. Thus such a metal, bonded at high temperature to a ceramicsurface, would, upon cooling, tend to peel ofi or bend away from thebonded surface, pulling away' portions of the ceramic with it. 'Ihis isavoided in the embodiment disclosed in FIG. 3 by the expedient ofproviding the terminals with relatively thin ilange portions la, r'n,and by bonding back-up or balancing ceramic members 5l, 5f?, of the samematerial as the disc il, to the Outer surfaces of the flanges 16a, 17a,at the same time and in the same manner 4that the flanges lla, ln, arebonded to the center ceramic disc il. This causes the bending stressesset up in the flanges 16a, 17a because of the bonding at the innerinterfaces to be balanced by similar but opposite-acting stresses set upbecause of the bonding at the outer interfaces.

As may be indicated by the requirements of size, current-rating, numberof capillaries, etc., i may supplement or replace the bonding of theterminal members to thc ceramic body by mechanical clamping means, solong as a vapor-tight seal is maintained at the interfaces.

ln FlGS. 4 and 5, l have shown embodiments of my invention in which thecentral ceramic disc and the terminal members have matching thermalcoefficients of expansion. in these forms, the back-up or stressbalancing ceramic rings 5l, 52, of the form of FIG. 3 are omitted, andthe flanged construction of the terminals l5, il? is replaced by asimple cylindrical structure. This simplifies the assembly and alsoincreases the strength of the terminal members.

in FlG. 5, the terminal members ld, Ii are constructed of titanium, andthe insulating disc is constructed of a ceramic material having the samecoelcient of thermal expansion as that of titanium. Such a material isavailable commercially under the trade name of Forsterite, from GeneralElectric Compan', supra. Th other features of construction of this form,including 13 the bonding technique, may be similar to those of the formof FIG. 3.

ln FIG. 5, the insulating disc l1 is constructed of a type of ceramicmaterial as described in connection with FIG. 3. The terminal members16, 17, however, are constructed of a conductive material comprising amixture of ceramic and metal, and known to the trade as a cermetmaterial, having a thermal coeilcient of exparisien about equal to thatof the ceramic disc 1l.

In this case, since ceimet materials cannot be satisfactorily tanped toreceive a closing screw such as screw 20 of FIG. 3, a differenttechnique of sealing is employed. ln accordance with this aspect of theinvention, the cermct terminal member 17 is provided with an axiallyextending hole or bore b by suitable means, such as by drilling, andalso with a transversely extending hole 17e which connects with the hole15b. rThe assembly is evacuated and filled with mercury in the mannerpreviously described and the assembly is sealed in the following manner.

A closely fitting aluminum pin S4 is driven into the hole lc so as toextend across the end of the hole liSb. A pair of closely fitting steelpins are then driven into the hole 53 from each end thereofsimultaneously, compressing the aluminum pin and the trapped mercury andcausing a portion of the aluminum pin to be extruded slightly into thehole 15b. This provides a positive vapor-tight and mechanically strongseal.

In the form of the invention shown in FlG. 6, the metallic terminalmembers 16, 17 are held to the ceramic body lili by mechanical clampingrather than by bonding. ln this connection it must be noted that suchmetallic members cannot be clamped directly against the ceramic body,since it is not possible to produce perfectly planar surfaces andcracking or" the ceramic will occur during clamping or in use.Accordingly, in this form, the metallic terminal members lo, i7, areclamped by bolts 55 against the ceramic disc ll, with resilient O-ringseals 5d therebetween at each interface, being seated in circulargrooves in the corresponding faces of the terminal members. T he boltsS5 are insulated from the terminal member lo by means of a spacer 57 andinsulating sleeves 58, and are threadedly engaged in terminal member i7.A generally ring-shaped or torroidal insulating spacer' 59, ravingopenings for the bolts 55, serves to space the terminal members i6, 17so that when they are clamped thcreagainst by the bolts 55, only apredetermined force is exerted upon the ceramic body ll.

The construction ot the assembly, including capillaries llZ, recess ld,axial opening l5 and plug-screw 2d, is generally similar to thatdescribed in connection with FIG. 3.

At lilG. 7 l have shown a combination circuit breaker and currentlimiting device integrally mounted in a unitary casing for connection inseries circuit relation to illustrate a preferred circuit aspect of myinvention. lt will be obvious, of course, that the invention may beembodied in other protective circuit arrangements connecting anelectrical load with a source of electric current supply, and mayinclude other switching or control devices as will be furtherillustrated hereinafter. lt will also be evident that, while FIG. 7shows a single pair of contacts and only one current limiter, a separateseries-connected combination of these devices would be used in each lineof a polyphase circuit. rihus for a three-phase line the circuit breakerof FlG. 7 represents a three-pole breaker only one pole of which isshown.

At FlG. '7 the combined protective device comprises an insulating casingincluding a base dil and a cover 6i, containing therein a circuitbreaker portion 62 and a current limiter portion o3. 'lf he circuitbreaker portion comprises a `lxcd Contact 64 connected to a plug-in typeline terminal 65 and a cooperating movable contact 66, one cooperatingpair for each pole, the movable contacts being mounted upon a pivotalymounted contact cross bar 67. The movable contacts 66 are actuatedmanually by a 14.. handle 63 through an overcenter spring type operatingmechanism indicated generally as 69. Such a mechanism is shown in Patent2,908,782, Kiesel. As will be more clearly evident from the foregoingpatent, the operating mechanism 69 is held in its closed circuitposition by a latch 7@ pivotally mounted at 7tlg and is biased to moveto a tripped open-circuit position upon release of the latch.

For the purpose of moving the latch 70 to releasing position in responseto predetermined current conditions in any of the poles, each pole isprovided with a combination thermal-magnetic tripping assemblycomprising an elongated bimetallic strip 7l, rigidly attached at one endto a conductive strip 72 which is mounted upon the casing cover 6l. Thelower end oi the bimetallic strip 71 is connected by a flexibleelectrical conductor or braid 73 to the movable contact 66. The trippingassembly also comprises magnetic-operating means comprising a magnetieldpiece 74, rigidly attached adjacent the lower end of the bimetallicstrip 7l, and an armature member 75 comprising an elongated memberpivotally supported at its end 75a in generally V-shaped bearingrecesses in the cover 6l. rl`he armature 75 has its lower end bent atright angles to its length having a down-turned end engageable with thelatch 7l) and having a notch through which the biinetallic strip extendsand providing a shoulder '76 in engagement with the right-hand side ofthe bimetallic strip as viewed. rlhe armature is constantly biased tothe left, as viewed, by tension spring 77 attached at one end of theterminal strap 72. Movement of the lower portion of the bimetallic stripto the right moves the armature member 7 5 to the right into engagementwith the latch 7. The armature member 75, however, when attracted by theiieldpiece 74, may move to the right as viewed independently of thebimetallic strip 7l to cause tripping.

For the purpose of facilitating connection of an outgoing electricalconductor (not shown), each pole is provided with a load terminal 7Srigidly mounted together with a fuse-clip type socket member 79, to thebase 6@ by means of screw 8d. The lower end of conductive strap 72 isalso provided with a fuse-clip type socket Si rigi ly attached theretoby suitable means, not shown. For the purpose of limiting short-circuitcurrents through the circuit protective device, there is provided acurrent-limiting assembly or device indicated generally at 63, havingendwise extending generally cylindrical terminal members 82, d3, held inelectrical plugged-in engagement in the clips 79, 31, respectively. Aremovable insulating shield fdd is provided for each current limiter 63.Each currentlimiting device e3 is of a type described hereinbefore inconnection with FIGS. l to 6.

ln the protective device of FIG. 7, the thermal and the magnetic tripelements each respond to overcurrent conditions to open the circuitbreal-:er contacts when the currents drawn by the load circuit exceedrated currents and each type of trip element contributes certaindesirable characteristics to the circuit interrupting capabilities oithe circuit breaker. For example, due to their inherent tiniedelayedaction, the biinetallic elements prevent unnecessary ser ticeinterruption on normal inrush currents, but continuous overloads willcause the bimetals to deflect thus releasing the trip latch and openingthe contacts. @n the other hand, 'the eiectromagnets 7d provide a morerapid circuit interruption in the'event of heavy ovcrloads. Excessivecurrent through one o' the electromagnets 7d will attract the associatedmovable armature 75 to trip the circuit breaker in the same manner. ltis to be noted that the electromagnets operate independently of thebimetallic elements and that each type of trip element is capable byitsell o opening the contacts. i

in practice, of course, the structure of the circuit breaker may differfrom the escribed above for illustrative purposes. What is important tonote in connection with FIG. 7 is that in the combination therecontemplated the breaker should have certain protective circuitinterrupting capabilities of its own to protect against overloadcurrents.

By reason of the two types of current responsive elements employed inthe breaker of FIG. 7, such a breaker is said to have a thermal-magnetictrip characteristic. rThe current limiters 63 aid further to protect notonly the load and supply circuits but also the breaker against shortcircuit currents which exceed the circuit interruptiriU capabilities ofthe breaker or which occur too rapidly for the breaker to respond tothem. These current limit rs do not of themselves interrupt the circuitas do current limiting fuses, but function instead to limit potentiallydestructive currents in order that the more slowly responding circuitbreaker may then interrupt the circuit at its contacts. Thiscurient-suppresion-withoutextinction action is an extremely importantaspect ot the invention, since it prevents the current limiter fromreestablishing its low impedance condition before the circuit breakeroperates sutiiciently. lt will be observed also that this action inaddition insures that the circuit will not be interrupted withoutactuation of the circuit breaker to its tripped condition.

In FG. S is illusrated on logarithmic coordinates the time vs. currenttrip characteristics, represented by a heavy line curve A, of a 200 amp.circuit breaker having thermal and magnetic trip characteristics. Thetime scale on these coordinates is represented in seconds while thecurrent scale is calibrated in hundreds oi root-mean-square (KMS.)symmetrical amperes. It is to be noted that the breaker tripcharacteristic has twoprincipal parts to it, an upper part slopingdownward to a knee of the curve at s: and a lower part dropping nearlyvertically until it levels out parallel to the x axis. The upper part ofthe curve above the knee is due to the inverse time-current tripcharacteristics of the thermal trip elements such as bimetals 7l in FlG.8, while the lower portion ot the curve is contributed by the magnetictrip elements such as the electromagnets 74 in FIG. 8. It is apparentthat this curve is a combination of two intersecting curves whichcomplement each other. The thermal trip characteristic provides a slowresponse to prolonged overloads not too greatly in excess of the ratedcurrent. For example, overload currents of 800 root-means-squaresymmetrical amperes, that is, four times the rated current, would causethe 'thermal elements to trip the circuit breaker in about 60 seconds.For more rapid tripping on much larger overload currents the magnetictrip characteristic below the Aknee of the curve indicates that thebreaker will interrupt the circuit in about .025 second on overload orshort circuit currents larger than approximately 2G00 amperes or tentimes the rated current.

lt will be observed from this graph (FlG. 8) that, in general, thehigher the excess current through the device, the shorter is the timerequired for the circuit breaker to interrupt the circuit automatically,It will also be observed, however, that the speed of opening ot thecircuit breaker portion reaches a lower limit at about 4000 amperes.Thus, no matter how high the excess or shortcircuit current may rise thecircuit cannot operate to interrupt the current in less than about .023second. This due to the fact that although the magnetic tripping action,which frees the mechanism to allow automatic opening, operatespractically instantaneously, nevertheless :the parts ot the mechanismincluding the movable contact :members d6, 67 have sutcient inertia thatthe operating :.spring, which has only a fixed amount of stored energy,icannot move them at a greater rate of speed.

it will also be observed that the time values indicated in connectionwith curve A represent time to interrupt 'the circuit. Such completeinterruption of ..90 electrical degl'ce, althugh complete interruptionmay not occur in less than about 490 electrical degrees or .O23 second.

Circuit breakers of different design and continuous current ratings willhave different opening times, o t course. in general, however, thefastest acting of such breakers requires at least about .013 second tointerrupt overload currents, i.e. about 280 electrical deg ees. A shortcircuit current, it should be noted, may be expected to reach its firstmajor crest in electrical degrees or less.

The general characteristic ot circuit breakers by which, on shortcircuit currents, their tripping mechanism is actuated practicallyinstantaneously, their contacts separate a small amount in less than 90electrical degrees, and complete interruption oi the current is achievedonly at some later time such as about ll/z cycles, is an importantconsideration to be kept in mind in connection with the operation of.the complete combination.

Within the sminimum time response of the circuit breaker very largelet-through currents might be permitted to ilow if no provision weremade to limit them. For the purposes of limiting let-through or surgecurrents it has been customary, as has been explained, to employ currentlimiting fuses in series with a breaker. Characteristics for possiblycoordinated current limiting ruses which might be matched with thebreaker are plotted as curves B, C and D. Desirably a current limitingfuse should, if properly coordinated with the breaker, accomplish twoends. First it should limit the let-through currents which occur beforethe circuit breaker can trip to the -lowest value possible. Second itscharacteristic should not cross the knee of the characteristic curve forthe breaker, since nuisance tripping would then be the result causinginterruptions of the circuit on short overload currents which the systemIis designed to allow. Because of the negative slope of the currentlimiting fuse char acteristic curves B, C, and D, a compromise must bemade between these two requirements. In practice it is customary toselect a coordinated current limiting fuse whose characteristic curvecornes as close as possible to the knee of the circuit breakercharacteristic curve without actually crossing it, and to acceptwhatever let-through value of current results.

Curve C represents the characteristic curve vfor an 800 ampere currentlimiting fuse `which is actually available commercially for use with the20() amp. circuit breaker whose characteriistic curve is represented bycurve A. With this fuse the minimum let-through 4current valuerepresented by the intersection of curve C with curve A is 11,000amperes or 55 times the rated current of the breaker. In practice,however, since characteristic curves do vary among selected currentlimiting fuses of the same nominal rating, to prevent nuisance trippingdue to an unfortunate mismatch between characteristic curves selectedtoo close to each other, a larger current limiting fuse is oftensubstituted. Curve D represents the characteristic of a 1200 amperecurrent limiting fuse which may also be employed with the 200 amperecircuit breaker. lts characteristic curve misses the knee of curve Awidely, and consequently, the let-through currents permitted to llow aremuch lange Th minimum let-through value for the 1200 ampere currentlimiting `fuse -is represented by the intersection of its curve D withcurve A and is 18,000 amperes or 90 times the rated current of thecircuit breaker. -From this it can be seen that the selection of anappropriate current limiting device is a very serious problem indeed.

lCurve B is a characteristic curve based upon calculations I have madeof an ideal current limiting fuse constructed according to the bestprinciples known today. A current limiting fuse to match thecharacteristic curve represented by curve B and to coordinate with a 200ampere circuit breaker is not available commercially and may never beavailable. According to my calculations even this perfectly matchedcurrent limiting yfuse would 17 let through surge currents of at least4800 amperes or 24 times the rated current of the circu-it breaker.

By contrast curve E represents the characteristic of a matched cu-rrentlimiting device constructed according to the principles of the presentinvention. The current limiter curve E is idesigned to just miss theknee X of curve A in the same manner as a matched fuse curve such `as B,but its closeness at this point is not critical as it is with a fuse. Ifthe fuse should melt at this point, it causes a nuisance interruptionand this may occur in a fuse as a result of repeated thermal experiencesin this region of the characteristic. With the current limiter of myinvention any occasional momentary limiting action at this point of thecharacteristic does not necessarily interrupt the circuit or requirereplacement of a component. Even if the breaker does interrupt it, itneed only be reclosed. Curve E, in just missing the knee of curve A,crosses the breaker trip characteristic curve at 3000 amperes or 15times the rated current of the breaker. It is therefore evident thatcurrent limiting devices constructed according to my invention effectsigniiicant irnprovements over other known devices intended toaccomplish somewhat similar purposes. In terms of the illustratedcharacteristic curves alone, one of the major advantages of thisinvention is drastically limiting potentially destructive currents as aconsequence of the relatively steep slope of the characteristic curve ofthe current limiting device as contrasted with the characteristics ofconventional current limiting fuses. The steepness of this slope is duein large part tothe fact of the small heat sink provided in my currentlimiting device. The characteristic curves, however, cannot tell thewhole story because they do not reveal that current limiting fuses aredestroyed in the act of their rst current limiting operation, whereascurrent limiting devices of the nature `described herein are capable ofrepeated operations.

`Incidentally the relative sizes of heat sinks in current limitinglfuses and in current limiters constructed according to this inventionshould not be confused with their thermal efficiencies. Conventionalcurrent limiting ruses have large heat sinks provided by the largeamount of vaporizable metal employed, and relatively low thermaleiciencies as a consequence of the sand or other granular insulating arcquenching material employed. ,In the practice of this invention, on theother hand, the heat sink provided by lthe vaporizable metal iscomparatively small, while the thermal eiciency of the device indissipating the energies released within it is relatively high due tothe proximity of massive terminals, the translucence of the ceramicdisc, and the employment of a large number of parallel-connectedcapillaries.

It remains -to be pointed out how a current limiting device constructedas described herein affects the circuit in which -it is placed duringfaul-ts which tend to produce destructive overcurrents. In FIG. 9 isillustrated in dotted lines the envelope of a prospective 601 cycleshort circuit current which at its peak would produce 60130 yamperes inthe circuit. The solid line is a copy of an oscillographic wave traceshowing the limited currents which were actually permitted to ilow as aresult of the presence in the circuit of a current limiter device of thesame general type shown in FIG. 1 in combination with a high speedmagnetic trip circuit breaker. At 29 degrees on the upward slope of theshort circ-uit current curve the conductive material in the capillariesof the current limiter device vaporized and sharply reduced the currentfrom the value of about 2630l amperes which it had reached down to anaverage level which, in comparison with the available 60:00' amperescale, was dicult to measure. It will be observed from the graph thatthe peak let-through current is` not a sharply pointed apex on thecurve, but rather, is curved. This is believed to be due to the rate ofvaporization of the conductive material within the capillaries. Thus, itis believed that the material forming the conductive segments is not allVaporized in the same instant of time. The resistance of the vapor stateis rather inserted at a certain rate over a finite space of time. Thiseffect tends to limit the inductive surges of voltage which might occurin the circuit due to the extremely rapid rates of change of current. Iffurther reduction of such inductive voltages is desired, the shape ofthe capillaries may be modified to provide a minimum diameter `at themid-point of the capillary, widening in conical fashion to a greaterdiameter at each end of the capillary.

The circuit breaker employed in obtaining the wave trace of FIG. 9 wasspecially arranged to have an extremely rapid magnetictripcharaeteristic. At 124 degrees, after the peak of the availableshort circuit current was passed, the circuit breaker succeeded inopening its contacts and the current which had been limited by thecurrent limiting limiting device fell to Zero. The current limiter hadtherefore succeeded in severely limiting the let-through current andthen reducing it to a value within Athe current interrupting ability ofthe circuit breaker.

The maintenance of the current in the circuit at a low level during thetime interval between the time when the magnetic trip device of thecircuit breaker is actuated Iand the time when the circuit breakerseparates its contacts suiiiciently to interrupt the circuit contributesimportantly to the successful operation of the combination of thecurrent limiter with the circuit breaker, since in this way the powerreleased in the circuit during this interval, `and particularly `in thecurrent limiter and in thecircuit'breaker, is maintained at a low level.If the current were not so reduced and regulated during this time, thepower released would place an appreciably greater burden upon thecurrent limiter, the circuit v breaker, and other components in thecircuit. Likewise, if

the current limiter were permitted to go into an oscillatingconducting-non-conducting condition, the total power released wouldIalso `be very high, since in each oscillation the current would be:permitted to rise to undesirably high levels.

It is significant to note that the value of the current in the regulatedor suppressed portion of the graph of FIG. 9, following the initialincidence and reduction' of a short circuit current, was determined tobe below even the normal current lrating of the circuit breakeremployed. The circuit breaker is therefore not called upon to perform-any more difficult operation than it performs fwithin its normalrating. It should be noted of course, that while the reduced `andregulated current of FIG. 9 is in itself not sufficient toI trip thecircuit breaker, the maximum let-through current peak is sufficient toinitiate magnetic `tripping operation, which action is self-completingdue to the inertia of the mechanism. Circuit breakers in the past havebeen necessarily designed to be able to interrupt the extremely highvalues of current occurring in short circuit conditions, although thereis only a slight possibility that they will ever be called upon to doso. The present invention makes this unnecessary and permits substantialsimplification and consequent economy in the design and construction ofsuch devices for any given current rating.

In FIGS. 1()l and 11 I have shown a combination in which thecurrent-limiter is used to supplement a simple switching device, toreduce arcing and to greatly increase interrupting capacity.

In FIG. 12 I have shown a reproduction of an oscillogram showing thetype of action obtained with the combination of FIGS. 1G and 11.- Inthis case, the currentlimiter 9i) is constructed t0 have a tiring pointjust below the normal current value of the circuit. The current-limiter94) is, however, normally by-passed by the contacts 91, 92, 93, so thatit carries only a very small portion of the current. As the Contact 91moves toward open position, however, an interim condition exists, inwhich the entire current passes through the current-limiter. Thischange-over occurs at point a on the graph.

No arcing occurs at this point because the currentlimiter is in its lowimpedance state. During the time between points a and b, the entirecurrent passes through the current-limiter and acts to increase thetemperature of the conductive -rnetal in the capillaries thereof. Atpoint b, the limiter tires and reduces the curre-nt to a much lowerlevel. The limiter also functions to maintain the current at this lowerlevel until lfurther move- -ment of the contact 91 causes it to`separate from contact 92, interrupting the circuit.

The graph of FIG. 12, moreover, indicates that the current-limiter ofthe present invention, in addition to its ability to operate to limit`and reduce high short-circuit currents, is usable as an inverse-timecurrent-responsive device to permit the flow of predetermined currentsfor predetermined times, and to thereafter greatly reduce such currents.In this connection, I therefore contemplate the use of thecurrent-limiter as an arc-reducing device in connection with electriccurrent switching functions generally.

The description contained herein is, of course, intended to beillustrative in nature and should not be considered as exhaustive orlimiting with respect either to the details of construction of thecurrent limiting device or to the possible arrangements for which it isadapted in particular circuits. The number of conductor-iilledcapillaries in the current limiter may be large or small depending onthe current-carrying requirements of the system but they should be assmall in cross-section as possible consistent with economy ofconstruction. It is not necessary that the capillaries be geometricallyparallel for I have contemplated that they might also radiate outwardlyfrom an electrode in the center of a disc or sphere and have constructedoperative samples in certain forms of this nature. Particularly at lowercurrent ratings a reinforcing structure, such as the hydraulicallypressurized housing I have shown, may not be necessary. For example,with a device having a single mercury-filled capillary .180 in. long and.0134 in. in diameter at the center of a disc of Lucalox of 1/2 inchdiameterand having stainless steel electrodes, without any clamping orhydraulic reinforcement to lend increased rigidity to the enclosure, Ihave successfully and repeatedly limited available currents of from 200to 677 amperes at about 176 volts. In `fact the physical configurationswhich the current limiter may take can be very diverse, as any worker inthe 'art will recognize, `and its precise physical form should not placeit beyond the scope of this invention ias long as it employs the basicprinciples described herein. Furthermore, although the invention isshown embodied in a circuit making use of a current limiter device and acircuit interrupting -device, it obviously may be used in circuit withinterrupters other than those illustrated or in circuit with otherimpedance devices in the event that current reduction withoutinterruption is adequate for the contemplated purpose. It should thus beapparent that various changes and combinations may be made inlaccordance with these teachings without departing in spirit or in scopefrom the invention as set forth in the appended claims.

What I claim as new `and desire to secure by Letters Patent of theUnited States is:

l. A current limiting device for gener-ating high pressure currentlimiting arcs comprising: a non-porous insulator having at least oneelongated :capillary aperture extending therethrough; a metallicconductor filling said capillary aperture from end to end; electricalterminal members in electrical contact with opposite ends of saidconductor-nlled aperture; Va rigid enclosure completely enclosing saidinsulator and said terminal members; a

hydraulic fluid under high pressure Within said enclosure surroundingsaid insulator and said terminal members to inhibit elastic deformationof said insulator and members; electrical connection means extending tothe outside of said enclosure from said terminal members for connectingsaid device in an electrical circuit requiring overcurrent protection,whereby currents in said capillary aperture in excess of a predeterminedvalue cause vaporization of the conductive material in said aperture andthe generation of high pressure current limiting arc therein.

2. A current limiting `device for generating high pressure currentlimiting arcs comprising: a non-porous insulator having at least oneelongated capillary aperture extending therethrough; a metallicconductor lling said capillary 'aperture from end to end; electricalterminal members in electrical contact with opposite ends of saidconductor-filled iaperture `for connecting said device in an electricalcircuit requiring overcurrent protection; means enclosing said insulatorand said terminal members and exerting thereon an initial subst-antialpressure inhibiting elastic movement of any portion thereof, wherebycurrents in said aperture in excess of a predetermined value causenon-expansive vaporization of conductive material in said aperture andthe generation of high pressure current limiting :arcs therein.

3. A current limiting ydevice for generating high pressure currentlimiting arcs comprising: a non-porous translucent ceramic insulatorhaving a plurality of elongated capillary apertures extendingtherethrough, the total surface area of the capillary apertures beingrelatively large compared to the volume enclosed by said apertures; ametallic conductor lling said capillary apertures from end to end; rigidelectrical terminal members in electrical contact with opposite ends ofsaid conductor-filled apertures for connecting said device in anelectrical circuit ree quiring overcurrent protection, said terminalmembers being relatively massive compared to the total volume of theconductor within said apertures; means enclosing said insulator and saidterminal members and exerting thereon Aan initial substantial pressurelinhibiting elastic movement of any portion thereof; whereby currents insaid capillary apertures in excess of a predetermined value cause thevaporization of conductive material in said capillary apertures and thegeneration of high pressure current limiting arcs therein, the energydeveloped by said arcs being dissipated by radiation through saidtranslucent ceramic insulator, by conduction into said relativelymassive terminal members, and by absorption through the relatively largesurface area of said capillary apertures.

References Cited in the file of this patent UNITED STATES PATENTS736,127 Mies Aug. 11, 1903 897,852 Sachs Sept. 1, 1908 1,316,095Illingworth Sept. 16, 1919 1,678,187 Illingworth July 24, 1928 2,306,728Heddaeus Dec. 29, 1942 2,358,215 Darling Sept. 12, 1944 2,895,031Kozacka July 14, 1959 2,920,241 Jacobs et al. Jan. 5, 1960 FOREIGNPATENTS 327,961 Great Britain Apr. 1'1, 1930 OTHER REFERENCES Suits:Measurement of Some Arc Characteristics at 1000 Atmosphere Pressure,Journal of Applied Physics, volume l0, March 1939, pages 203-206 cited.

2. A CURRENT LIMITING DEVICE FOR GENERATING HIGH PRESSURE CURRENTLIMITING ARCS COMPRISING: A NON-POROUS INSULATOR HAVING AT LEAST ONEELONGATED CAPILLARY APERTURE EXTENDING THERETHROUGH; A METALLICCONDUCTOR FILLING SAID CAPILLARY APERTURE FROM END TO END; ELECTRICALTERMINAL MEMBERS IN ELECTRICAL CONTACT WITH OPPOSITE ENDS OF SAIDCONDUCTOR-FILLED APERTURE FOR CONNECTING SAID DEVICE IN AN ELECTRICALCIRCUIT REQUIRING OVERCURRENT PROTECTION; MEANS ENCLOSING SAID INSULATORAND SAID TERMINAL MEMBERS AND EXERTING THEREON AN INITIAL SUBSTANTIALPRESSURE INHIBITING ELASTIC MOVEMENT OF ANY PORTION THEREOF, WHEREBYCURRENTS IN SAID APERTURE IN EXCESS OF A PREDETERMINED VALUE CAUSENON-EXPANSIVE VAPORIZATION OF CONDUCTIVE MATERIAL IN SAID APERTURE ANDTHE GENERATION OF HIGH PRESSURE CURRENT LIMITING ARCS THEREIN.